Precision 954G Vacuum Tube Checker

This Web page last updated date Jan 22 2010.

This Web page is about the Precision 954-G Vacuum Tube Checker, by Precision Apparatus Company of Elmhurst NY. It's part of our Web page support for old computer repair and restoration. We support a lot about old computing and other retro-technology, see my Web site home page.

If you have any corrections to this information I'd appreciate hearing them: please include appropriate references so I can confirm that info. I am not responsible for errors or omissions in the above info, use it at your own risk.

Acquisition

In April 2009, I went to a hamfest where, long story short, a guy gave me a somewhat chewed up tube tester. (Either that, or I paid $10 for it - practically free.) Looks to me like it sat in some basement for a few years. The dovetail joints were all loose. Also, the lid was homemade from plywood, not well done. But opening it up, it looked like the components were in good shape. The front plate identifies it as a model 934G, serial number about 1200.

Precision Apparatus Company of Elmhurst NY apparently built a number of vacuum tube testers. A number of Web sites represent the history of these testers, with photos of them in very good condition. Some offer documents and tube lists; a few people offer reprinted rolls. Units of this sort and brand seem to sell for several tens of dollars in 2010; less if in this condition or without documents, more if very clean and undamaged.

repairs

In Jan 2010, I cleaned up and checked out the tester. All the components looked OK. I dusted the case and interior with a dry paintbrush, then used water with a little dish soap and a toothbrush to clean the panel. (Harsher cleaners could remove the painted lettering, etc.). It cleaned up fairly well, but it's clear the unit was well used. The case is pretty beat, the wood is cracked, but it is otherwise sound. The replacement top, as noted above, is not very good. The interior of the case is dusty but OK. The photo shows an old C-cell battery which still functions! I found a manual for it on the Web and downloaded it for operational instructions and the schematic. The roll chart in this unit seems to be OK, the plastic protection for it has held up OK. A Web search for the model will find these resources.

I used a Variac to reduce the AC voltage, and powered up the unit. It's a little compliated to get started, because the AC power switch is interlocked with the pin selection buttons (A through J). The mechanism was a little stiff, but some light oil (I use sewing machine oil) and some exercise of the buttons got the mech to operate. I applied power at a very low voltage to test the "line operation" feature. Most of these testers adjust AC voltage to the transformer, using the meter at its 50% setting.

There was initially no response to AC power. I found one AC power wire was broken off the AC switch, and soldered that in place. The meter then responded at low voltage! I ran the unit off and on at about 30V AC, to make sure any capacitors or ancient rectifiers would "form" correctly. Then I brought the AC line voltage up and adjusted the AC line rheostat to 50% level. I had to clean the power lamp contacts for it to operate. I selected a 6AQ5, set the buttons and knobs, and the 954G tested it as "good". ALso, the "magic eye" tube operates as a "short indicator". In the photo it's showing a "short" for the filament.

Technical

The operation of the tester is kind of busy. As you can see by the schematic, this seems to be an "emissions" tester, but I'm not exactly sure of that. A roll chart, or supplementary chart, shows for each vacuum tube which connections to make and what load to set, and the filament voltage. Filament voltage is set by selecting a secondary tap of the correct voltage. The vacuum tube is set up as a diode, all grids connected to the plate.

The transformer has taps to set the filament voltage. At a given voltage and load, a tube under test is evaluated as a diode. It should draw a given current based on DC voltage and cathode emission. The meter indicates the current as a percentage of "new". As the tube ages, the cathode emits less, the current drops. Eventually the filament breaks, emission is insufficient, or shorts or opens occur between elements. Shorts and opens can be tested by disconnecting the grids (reduces current unless open) or by shorting them to ground (stops current) respectively. This particular brand and model tester, can also read DC voltage, current and ohms, as a simple analog "VOM".

Further repairs

I ought to fix the joints, I presume using white glue. The top ought to be replaced, but only if I can build something with the correct joints. I could use a pair of hinges! The top half of each hinge is not original, and I can't find this kind of hinge. I'm afraid I'll have to hack a pair of conventional hinges to get what I want. I can do that but they may not look like the originals. Anyone have a clue about getting a pair of these? It's possible, however, this might become a parts or repair resource for a unit in a better case.

Herb Johnson

Copyright © 2010 Herb Johnson